1. Field of the Invention
This invention relates to hydrogen-functional polymethylsilsesquioxane, to a method for its preparation, and to curable compositions that use said reactive group-functional polymethylsilsesquioxane.
2. Description of the Prior Art
Silicone resins that contain 1.5 oxygen atoms per silicon atom are generically known as polyorganosilsesquioxanes. Polymethylsilsequioxanes are highly heat resistant and exhibit good electrical insulation properties and flame retardancy, and this good property spectrum has resulted in their use as resist materials and interlayer dielectric films in semiconductor fabrication (see, among others, "Shirikoon Handobukku" (English title: Silicone Handbook), edited by Kunio Itoh, published by Nikkan Kogyo Shinbunsha (1990)).
Methods are already known for the synthesis of polymethylsilsesquioxanes. For example, polymethylsilsesquioxane can be synthesized by dissolving methyltrichlorosilane in the presence of amine in a single solvent or mixture of solvents selected from ketones and ethers, adding water to this system dropwise to effect hydrolysis, and then heating to effect condensation (see Japanese Patent Publication (Kokoku) Numbers Sho 60-17214 (17,214/1985) and Hei 1-43773 (43,773/1989) and U.S. Pat. No. 4,399,266). Another synthesis example is taught in EP 0406 911 Al and Japanese Patent Publication (Kokoku) Number Sho 62-16212 (16,212/1987). These references teach the dissolution of a trifunctional methylsilane in organic solvent; then hydrolysis by the dropwise addition of water to this solution at a temperature from -20.degree. C. to -50.degree. C. under an inert gas pressure of 1,000 to 3,000 Pa; and thereafter condensation by heating. Yet another synthesis example is disclosed in Japanese Patent Application Laid Open (Kokai or Unexamined) Number Hei 3-20331 (20,331/1991). This reference teaches the reaction in organic solvent of methyltriacetoxysilane with an equivalent amount of alcohol and/or water to synthesize the alkoxyacetoxysilane; polycondensation of the alkoxyacetoxysilane in organic solvent in the presence of sodium bicarbonate to give a prepolymer; and condensation of this prepolymer by heating in the presence of at least I catalyst selected from the alkali metal hydroxides, alkaline-earth metal hydroxides, alkali metal fluorides, alkaline-earth metal fluorides, and triethylamine. Still another synthesis example is found in Japanese Patent Application Laid Open (Kokai or Unexamined) Number Hei 3-227321 (227,321/1991). This reference teaches the dissolution of alkali metal carboxylate and lower alcohol in a mixed liquid system that forms two phases (water and hydrocarbon solvent); the dropwise addition of methyltrihalosilane into this system to effect hydrolysis; and condensation by heating.
The polymethylsilsesquioxanes afforded by these methods share a characteristic feature: they are hard but brittle. Some of the preceding references even include tactics for addressing this problem. Japanese Patent Publication (Kokoku) Number Hei 1-43773 instructs regulating the fraction with molecular weight.ltoreq.20,000 (molecular weight as determined by gel permeation chromatography (GPC) calibrated with polystyrene standards) to 15 to 30 weight % of the polymethylsilsesquioxane. However, even this does no more than enable the preparation of coatings with thicknesses of about 1.8 to 2.0 .mu.m. Similarly, the technology in EP 0 406 911 A1 can only provide coatings with maximum thicknesses of 3 to 3.5 .mu.m without cracking. At larger film thicknesses cracking occurs, and of course the flexibility that would permit the fabrication of an independent or stand-alone film is absent.
We have already discovered (see EP 786 489 Al and WO 9707164) that a coating that combines flexibility with high thermal stability is provided by the cure of a polymethylsilsesquioxane having a molecular weight and hydroxyl content in specific ranges and preferably prepared by a special method.
On the subject of the silylation of the residual silanol in polysilsesquioxane, a synthetic method for this is disclosed in, for example, J. Am. Chem. Soc., 1990, Volume 112, pages 1931 to 1936. Japanese Patent Application Laid Open (Kokai or Unexamined) Number Sho 61-221232 (221,232/1986) teaches a method for the preparation of silylated polysilsesquioxane comprising the use of silylating agent to terminate the reaction in the polysilsesquioxane synthesis method of EP 0 406 911 Al and Japanese Patent Publication (Kokoku) Number Sho 62-16212. Japanese Patent Application Laid Open (Kokai or Unexamined) Numbers Hei 6-279586 (279,586/1994), Hei 6-287307 (287,307/1994), and Hei 7-70321 (70,321/1995) teach that stabilization can be achieved and gelation can be avoided by trimethylsilylation of the hydroxyl in polysilsesquioxane in which methyl constitutes 50 to 99.9 mole % of the pendant organic groups and crosslinking-reactive groups are present in the remaining organic groups. However, even without silylation the polymethylsilsesquioxane disclosed by us in EP 786 489 Al and WO 9707164 does not gel during its preparation and can be stably stored at room temperature. Japanese Patent Application Laid Open (Kokai or Unexamined) Number Hei 5-125187 (125,187/1993) teaches that an increased storage stability can be obtained by trialkylsilylation of the hydroxyl in polysilsesquioxane having Mn.gtoreq.100,000 and methyl as 50 to 100 mole % of its pendant organic groups. The above-referenced Japanese Patent Publication (Kokoku) Number Sho 62-16212 also teaches that silylation of the hydroxyl in polymethylsilsesquioxane improves stability.
On the subject of the organic groups in silsesquioxanes, silsesquioxanes functionalized with various crosslinking-reactive groups are described in, for example, Chem. Rev., 1995, Volume 95, pages 1409 to 1430.